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newSchoolWorks.com › VoTechWorks › Agriculture, Environment & Public Service › Water / Wastewater Operator › Water and Wastewater Regulations › Lesson 3
AGR-WATE71-06 · Lesson 3 · 12.5 clock hours · theory

Surface Water Treatment Rule and Ground Water Rule Requirements

Objective. Students will describe the treatment requirements of the Surface Water Treatment Rule and Ground Water Rule, including filtration and disinfection performance standards and conditions that trigger additional action.

⚠ Pending professional review

This lesson, its worked examples and its practice questions has not yet been verified by a licensed professional in the field or by a certification or approval body. It is a study draft and may contain errors. Do not use it as instructions for work on a job site or on live equipment. Always follow your instructor, your supervising professional, and current official references and regulations.

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Why Federal Treatment Rules Exist

Drinking water drawn from surface sources or certain groundwater sources can carry pathogens that cause serious illness. Congress directed the U.S. Environmental Protection Agency (EPA) to set minimum treatment standards so that every community water system delivers water that is safe to drink. Two foundational rules govern these requirements: the Surface Water Treatment Rule (SWTR) and the Ground Water Rule (GWR). As a licensed operator, you are legally responsible for meeting these standards every day your plant is running.

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Surface Water Treatment Rule

The SWTR applies to all public water systems that use surface water (rivers, lakes, reservoirs) or groundwater under the direct influence of surface water (GWUDI). The rule was first published in 1989 and has been strengthened by later rules including the Interim Enhanced SWTR and Long Term 2 Enhanced SWTR.

Required Log Inactivation Credits

The SWTR uses a log credit system to measure how effectively treatment removes or inactivates pathogens. The baseline rule requires:

These credits come from a combination of physical removal (filtration) and chemical inactivation (disinfection). A conventional filtration system receives 2.5-log credit for Giardia removal from filtration alone; the remaining 0.5-log must come from disinfection. The disinfection credit is calculated using CT values — the product of disinfectant residual concentration (C, in mg/L) and contact time (T, in minutes). EPA publishes CT tables for chlorine, chloramines, chlorine dioxide, and ozone at various pH values and temperatures.

Filtration Requirements

Surface water systems must provide filtration unless they qualify for the filtration avoidance criteria, which require exceptional source water quality and watershed protection programs. Most systems provide filtration.

Turbidity is the primary indicator of filtration performance. Turbidity is measured in Nephelometric Turbidity Units (NTU). Under the SWTR:

When turbidity limits are exceeded, the operator must investigate immediately, document the event, notify the state, and take corrective action. Continued or repeated violations can result in a public notice requirement.

Disinfectant Residual Requirements

A detectable disinfectant residual must be maintained throughout the distribution system. At least 95% of distribution samples each month must show a measurable residual. Systems must also maintain a minimum residual at the entry point to the distribution system as defined by the state.

Safety Note for Disinfection Operations

Chlorine gas and hypochlorite solutions are corrosive and toxic. Always follow your facility's chemical hygiene plan: wear appropriate PPE (chemical splash goggles, face shield, chemical-resistant gloves and apron), ensure ventilation when handling chemicals, and never mix chlorine products with ammonia-containing compounds. Cylinders and bulk tanks must be stored per safety regulations. Emergency eyewash and shower stations must be accessible.

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Ground Water Rule

The Ground Water Rule (GWR), finalized in 2006, targets groundwater systems that are at risk of fecal contamination. Unlike the SWTR, it does not apply uniformly to all groundwater systems — it is risk-targeted.

Which Systems Are Subject?

The GWR applies to public water systems that use groundwater sources not under the direct influence of surface water. GWUDI systems are already covered by the SWTR. The GWR focuses on systems where there is a credible risk of fecal contamination reaching the aquifer.

Triggered Source Water Monitoring

A triggered source water monitoring event occurs when a system has a positive sample for a fecal indicator (such as total coliform or E. coli) in the distribution system. When this happens, the system must collect additional source water samples to determine whether the source itself is contaminated. The rule specifies sampling locations, timing, and the indicators to be tested. States may also require assessment source water monitoring for higher-risk systems on a routine schedule.

Significant Deficiencies and Corrective Actions

If source water monitoring confirms fecal contamination, or if a sanitary survey identifies a significant deficiency, the system must take corrective action. Approved corrective actions include:

1. Eliminating the source of contamination

2. Providing treatment that achieves at least 4-log treatment of viruses (the primary pathogen concern in groundwater)

3. Abandoning the contaminated source

4. Providing an alternate source

The system must notify the state and, if required, issue public notification to consumers.

Operator Responsibility Under the GWR

Operators must be alert to sanitary survey findings, respond promptly to positive indicator results, and maintain documentation of all monitoring, notifications, and corrective actions. Failure to act after a triggered monitoring event is a violation of the Safe Drinking Water Act and can result in penalties and enforcement action.

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Connecting the Rules to Daily Practice

Both the SWTR and GWR share a common logic: identify risk, measure performance, and take action when standards are not met. As an operator, your role is to monitor continuously, calculate CT values correctly, keep accurate records, and escalate issues to your supervisor and the state when thresholds are crossed. Accurate recordkeeping is not optional — it is the legal record that proves your system protected public health.

Key terms8

Surface Water Treatment Rule (SWTR)EPA regulation requiring systems that use surface water or GWUDI to provide filtration and disinfection meeting specified log-removal and turbidity standards.
Ground Water Rule (GWR)EPA regulation targeting groundwater public water systems at risk of fecal contamination, requiring triggered source water monitoring and corrective actions when fecal indicators are detected.
Log inactivation creditA measure of pathogen reduction expressed as a power of ten; 1-log = 90% reduction, 2-log = 99%, 3-log = 99.9%, and 4-log = 99.99%.
CT valueThe product of disinfectant residual concentration (C, mg/L) and contact time (T, minutes) used to calculate disinfection credit for pathogen inactivation.
TurbidityA measure of water cloudiness caused by suspended particles, expressed in Nephelometric Turbidity Units (NTU); used as a key indicator of filtration performance.
Groundwater under the direct influence of surface water (GWUDI)A groundwater source that receives significant infiltration from surface water, making it subject to the SWTR rather than the GWR.
Triggered source water monitoringUnder the GWR, additional source water sampling required when a fecal indicator is detected in a groundwater system's distribution system samples.
Significant deficiencyA finding during a sanitary survey indicating a condition that has the potential to cause, or is causing, contamination of the drinking water supply; triggers required corrective action under the GWR.

Worked examples4

1. A surface water system using conventional filtration (which receives 2.5-log credit for Giardia removal from filtration) needs to achieve a total of 3-log Giardia inactivation. The water temperature is 10°C and pH is 7.0. The CT table shows that free chlorine requires a CT of 165 mg/L·min for 0.5-log inactivation of Giardia at those conditions. The plant maintains a chlorine residual of 1.5 mg/L. What minimum contact time (T) is required to meet the disinfection CT requirement?
  1. Step 1 – Determine remaining log credit needed from disinfection: Total required = 3.0 log; filtration credit = 2.5 log; disinfection must provide 3.0 − 2.5 = 0.5 log.
  2. Step 2 – Find the required CT from the EPA CT table: For 0.5-log Giardia inactivation at 10°C and pH 7.0, the required CT = 165 mg/L·min (given).
  3. Step 3 – Apply the CT formula: CT = C × T, so T = CT ÷ C = 165 ÷ 1.5 = 110 minutes.
  4. Step 4 – Verify: C (1.5 mg/L) × T (110 min) = 165 mg/L·min, which meets the required CT. The system must provide at least 110 minutes of contact time.
Answer: The minimum required contact time is 110 minutes.
2. A filter plant takes turbidity readings every four hours throughout June (30 days, 180 readings total for the combined filter effluent). At month's end, 12 readings were above 0.3 NTU and 2 readings exceeded 1.0 NTU. Is the system in compliance with SWTR turbidity limits?
  1. Step 1 – Check the 95% compliance threshold: 95% of 180 readings = 0.95 × 180 = 171 readings must be ≤ 0.3 NTU. Readings ≤ 0.3 NTU = 180 − 12 = 168.
  2. Step 2 – Compare: 168 compliant readings out of 180 = 168 ÷ 180 = 0.9333, or 93.3%. This is below the required 95%.
  3. Step 3 – Check the never-exceed limit: 2 readings exceeded 1.0 NTU, which violates the absolute maximum of 1 NTU.
  4. Step 4 – Conclusion: The system is in violation of both the 95% monthly standard and the 1 NTU maximum. The operator must investigate, document, notify the state, and take corrective action.
Answer: The system is NOT in compliance — it fails both the 95% monthly standard (93.3% < 95%) and the never-exceed 1 NTU limit (2 exceedances). State notification and corrective action are required.
3. A plant needs to achieve 4-log virus inactivation using free chlorine. The CT table indicates that at the current water temperature and pH, achieving 4-log virus inactivation requires a CT of 12 mg/L·min. The system's contact time through the clearwell is 30 minutes. What minimum chlorine residual must the operator maintain at the entry point to the clearwell to meet this requirement?
  1. Step 1 – Use the CT formula: CT = C × T, so C = CT ÷ T.
  2. Step 2 – Substitute values: C = 12 mg/L·min ÷ 30 min = 0.4 mg/L.
  3. Step 3 – Interpretation: The operator must maintain a free chlorine residual of at least 0.4 mg/L at the entry point to the clearwell to achieve the required 4-log virus CT.
Answer: The operator must maintain a minimum free chlorine residual of 0.4 mg/L.
4. A groundwater system collects routine distribution samples. One sample tests positive for total coliform. Under the Ground Water Rule, what must the system do next, and what outcome determines whether corrective action is required?
  1. Step 1 – Recognize the trigger: A positive fecal indicator result in distribution triggers the GWR triggered source water monitoring requirement.
  2. Step 2 – Required action: The system must collect source water samples from each affected groundwater source according to state-specified protocol and timing.
  3. Step 3 – Evaluate results: If source water samples confirm fecal contamination of the source, the system must implement a state-approved corrective action (e.g., provide 4-log virus treatment, fix the contamination pathway, or use an alternate source).
  4. Step 4 – Documentation and notification: The system must notify the state of the positive result and all subsequent findings, and maintain records of all monitoring and corrective actions taken.
Answer: The system must conduct triggered source water monitoring. If source contamination is confirmed, a state-approved corrective action — such as providing 4-log virus treatment — is required, along with state notification.

The math in this lesson2

Learners rearrange CT = C × T to find required contact time from a 165 mg/L·min CT value.
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Brush up in the math halls at newSchoolWorks.com:
In class: Lecture and guided notes on the Surface Water Treatment Rule: required log inactivation credits for Giardia and viruses, turbidity performance standards at the filter effluent, and when a system must provide filtration. · Case study: given a surface water system's turbidity log and Giardia inactivation calculation results, students identify whether the system is in compliance or in violation and what steps the operator must take. · Small-group review of the Ground Water Rule: students identify which groundwater systems are subject to the rule, what triggers a triggered source water monitoring event, and required corrective actions; groups present findings to the class.
Materials: EPA Surface Water Treatment Rule and Ground Water Rule fact sheets, sample turbidity and CT calculation worksheets, case study handout, whiteboard
Check: Students correctly classify three scenarios as compliant or non-compliant and state the required operator action; instructor verifies accuracy of all three before marking complete.

Practice check8

Answer each question, then check it. Nothing is recorded; this is your own practice.

Treatment Process and Operations
1. The Surface Water Treatment Rule requires that conventional filtration systems achieve a minimum total removal/inactivation of Giardia lamblia of:
Regulations and Compliance
2. Under the SWTR, the combined filter effluent turbidity must not exceed 0.3 NTU in what percentage of measurements taken each month?
Treatment Process and Operations
3. A surface water plant's CT calculation shows that the current disinfection CT achieved is lower than the CT required for the target log inactivation. What is the most appropriate immediate operator action?
Treatment Process and Operations
4. Which of the following best defines a CT value as used in drinking water disinfection?
Regulations and Compliance
5. Under the Ground Water Rule, a triggered source water monitoring event is initiated when:
Regulations and Compliance
6. A groundwater system's source water monitoring confirms fecal contamination of one well. Which of the following is an approved corrective action under the Ground Water Rule?
Regulations and Compliance
7. A surface water plant records 200 combined filter effluent turbidity measurements in one month. Twelve readings exceed 0.3 NTU and none exceed 1.0 NTU. Is the system in compliance?
Source Water and Watershed
8. The SWTR applies to groundwater sources described as GWUDI. What does GWUDI stand for and why does it matter?
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